Filters have long been used in the detection of electrical signals. For example, in communications applications, such as microwave applications, it is desirable to filter out the smallest possible passband. In this way, it is possible to divide a fixed frequency spectrum up into the largest possible number of bands. Narrow bandpass filters typically have a fractional bandwidth (absolute bandwidth divided by the center frequency) of less than substantially 10%, and more preferably less than 5%.
Applications in the telecommunications field are of particular importance. As more users desire to use the microwave band, the use of narrow band filters will increase the number of users in the spectrum. Of most particular importance is the frequency range from approximately 800-2,200 MHz. In the United States, the 800-900 MHz range is used for analog cellular communications. Personal communication services are planned for the 1,800 to 2,200 MHz range.
Historically, filters have fallen into three broad categories. First, lumped element filters have used separately fabricated air wound inductors and parallel plate capacitors, wired together to form a filter circuit. These conventional components are relatively small compared to the wave length, and accordingly, make for a fairly compact filter. However, the use of separate elements has proved to be difficult in manufacture, resulting in large circuit to circuit variations. The second conventional filter structure utilizes 3-dimensional distributed element components. These physical elements are sizeable compared to the wavelength. Coupled bars or rods are used to form transmission line networks which are arranged as filter circuit. Ordinarily, the length of the bars or rods is 1/4 or 1/2 of the wavelength at the center frequency of the filter. Accordingly, the bars or rods can become quite sizeable, often being several inches long, resulting in filters over a foot in length. Third, printed distributed element filters have been used. Generally, they comprise a single layer of metal traces printed on an insulating substrate, with a ground plane on the back of the substrate. The traces are arranged as transmission line networks to make a filter. Again, the size of these filters can become quite large. The structures also suffer from various responses at multiples of the center frequency.
Historically, filters have been fabricated using normal, that is, non-superconducting materials. These materials have inherent lossiness, and as a result, the circuits formed from them have varying degrees of loss. For resonant circuits, the loss is particularly critical. The Q of a device is a measure of its power dissipation or lossiness. Resonant circuits fabricated from normal metals in a microstrip or stripline configuration have Qs at best on the order of four hundred. See, e.g., F. J. Winters, et al., "High Dielectric Constant Strip Line Band Pass Filters", IEEE Transactions On Microwave Theory and Techniques, Vol. 39, No. 12, Dec. 1991, pp. 2182-87.
With the discovery of high temperature superconductivity in 1986, attempts have been made to fabricate electrical devices from these materials. The microwave properties of the high temperature superconductors has improved substantially since their discovery. Epitaxial superconductive thin films are now routinely formed and commercially available. See, e.g., R. B. Hammond et al, "Epitaxial Tl.sub.2 Ca.sub.1 Ba.sub.2 Cu.sub.2 O.sub.8 Thin Films With Low 9.6 GHz Surface Resistance at High Power and Above 77.degree. K.", Applied Physics Letters Appl. Phy. Lett., Vol. 57, pp 825-27, 1990.
In theory, devices with zero resistance should have an infinite Q. However, even superconductive devices are not perfectly lossless at high frequencies. However, they do have exceedingly high Qs. For example, a thallium superconductor strip line resonator at 8.45 GHz has been measured with a Q of 26,000 as compared to a Q of literally a few hundred for the best conventional metal resonator. (See, Winter, et al., cited above.)
Various filter structures have been formed utilizing significant superconductive components. For example, a narrow band microwave filter is shown in plan view in FIG. 1, which structure is taken in substantially part from European patent application 0357507, subject to common assignment with the instant application. This narrow band filter is formed in a microstrip configuration, that is, one in which the conductor faces a dielectric environment which is not uniform around the conductor. In the structure of FIG. 1, the conductors generally are deposited upon a dielectric substrate and are only exposed to air above the conductors, thereby creating a difference in the dielectric constant above and below the conductors. In FIG. 1, connectors 10, having a width designated "w" on FIG. 1, serve as input and output connections to the overall filter 12. Generally, the filter 12 is symmetrical around the longitudinal center line of the center resonator 14. Intermediate resonators 16 are arranged with a 1/4 wavelength overlap (a) between each of the input 10 and center resonator 14. In this microstrip configuration, the bandpass filter characteristics are substantially varied by adjusting the spacing S between the center resonator and intermediate resonator 16 relative to the spacing "t" between the connectors 10 and intermediate resonator 16.
FIGS. 2a, 2b and 2c shows various prior art configurations of normal metal half wave resonators in a side coupled arrangement. Each of the filters in FIGS. 2a, and 2c show half wave resonators where the overlap to the nearest neighbor is substantially 1/4 wavelength. These figures show common methods of folding side-coupled half-wave filters. See, Harlan Howe, Jr., Strip Line Circuit Design, Microwave Associates, Burlington, Mass., Artech House, Inc., 1974, pp 217. The filters are described in the Howe article as in a pyramid configuration (FIG. 2a), a hairpin configuration (FIG. 2b), and a pseudo-interdigital configuration (FIG. 2c). The arrangements of FIGS. 2a, 2b and 2c are designed to overcome in part the perceived disadvantage of making a side coupled half-wave resonator as shown in FIG. 1, where the overall length can be excessive.
FIG. 3 shows a plan view of a 5-pole, interdigital microstrip filter layout disclosed in Schmidt, et al, "Measured Performance at 77 K of Superconducting Microstrip Resonators and Factors", IEEE Transactions on Microwave Theory and Techniques, Vol. 39, No. 9, September, 1991, pp 1475-78. Each resonator 30 is substantially 1/2 wavelength in length, and each resonator 30 is side coupled to its adjacent resonator. The resonators 30 are substantially aligned relative to one another. This structure will only work in a transmission line structure with unequal coupled line even and odd mode phase velocities (such as in a microstrip configuration) which gives rise to forward coupling.
FIG. 4 shows yet another prior art interdigital filter structure. Here, a 3-wire-line interdigital filter has 1/4 wavelengths overlap in 1/2 wavelength resonators. This structure is disclosed in Levy, "3- Wire-Line Interdigital Filters of Chebyshev and Eliptic Function Characteristic for Broad Bandwidths", Electronic Letters, Vol. 2, pp 13-14, December, 1966. This structure may be used in either a microstrip or strip line configuration. A strip line configuration is one in which the conductor is surrounded by a substantially homogenous dielectric environment. The disclosure of Levy is made with respect to non-superconducting filter structures.
FIG. 5 shows a prior art structure having an end coupled strip line filter formed from superconducting materials. This structure is generally symmetric around the center resonator 50. Intermediate resonators 52 are located between the center resonator 50 and end resonators 54 in an end coupled arrangement. Each of the resonators 50, 52 and 54 is preferably a half wavelength resonator, a 1/4 wavelength being designated ".lambda./4" in FIG. 5. A 50 Ohm input impedance is indicated. This structure is described in Matthaei et al, "High Temperature Superconducting 8.45 GHz Bandpass Filter For the Deep Space Network", presented at 1993 IEEE MTT-5 International Symposium, June 14-18, 1993, Atlanta, Ga., addressed at pages 1273-1276.
A need remains for compact, reliable narrow band filters. Despite the clear desirability of improved electrical circuits, including the known desirability of converting circuitry to include superconducting elements, room remains for improvement in devising alternate structures for filters. It has proved to be especially difficult to substitute high temperature superconducting materials in conventional circuits to form superconducting circuits without severely degrading the intrinsic Q of the superconducting films. Among the problems encountered are radiative losses and tuning, which remain despite the clear desirability of an improved filter. Additionally, size has remained a concern, especially for narrow band filters. For example, the end coupled strip line filters of FIG. 5 a 50 Ohm input impedance is indicated result in long thin structures, which present manufacturing difficulties when formed from superconducting films. Conventionally, the technique for forming narrow band side coupled resonator filter structures as shown in FIGS. 1, 2a, 2b, 2c, 3 and 4 is to space the resonators further apart in order to obtain small coupling coefficients between resonators. However, this approach has two distinct disadvantages, first, the structures become unreasonably large and second, as the distance between resonators increases, the coupling to structures other than adjacent resonators increases, resulting in losses and decreased performance. Additionally, power limitations arise in distributed structures.
Despite the clear desirability in the art for forming narrow bandpass filters to permit efficient use of the frequency spectrum, a need remains for improved designs capable of achieving those results in an efficient and cost effective manner.